The present invention relates to a water purification process in which nitrate ions are removed from an aqueous solution thereof and to a process for the removal and destruction of nitrate ions from water such as ground water or surface water.
The recent widespread use of fertilizers has lead to an increase in the level of nitrates in water. Levels in excess of 50 ppm in drinking water have been linked to health problems such as xe2x80x9cBlue Baby Syndromexe2x80x9d and possibly stomach cancer. Furthermore, nitrates are often present in effluent which can be discharged into the water system in concentrated form; such nitrate discharge has been identified as a major cause of algal xe2x80x9cbloomxe2x80x9d in reservoirs and also inland and coastal water eutrophication. This prevalence of nitrate in the environment has led to legislation limiting the permitted level of, nitrates in treated water and effluent.
Nitrates are currently commonly removed from solution either by ion exchange or reverse osmosis.
In an ion exchange process nitrate-containing solutions (typically containing calcium, magnesium and sodium cations of nitrate, sulphate, chloride and bicarbonate anions) are passed through a column containing an anion exchange resin. When the anion exchange resins are fully loaded with nitrate ions, the resin is regenerated, for example using a solution of brine (sodium chloride). Nitrate ions then exchange with chloride ions in the brine and the resulting sodium nitrate and brine mixture is then discharged as waste.
With reverse osmosis, nitrate solutions pass through a membrane which retains approximately 90% of the nitrate (and other) ions in, typically, 20% of the solution. The resultant concentrated solution of the retained ions must then be discarded.
Other technologies, such as bio-denitrification, are also available for removal of nitrate ions from solution.
A problem associated with known techniques of nitrate removal is that relatively concentrated nitrate solutions are discharged. Furthermore, in the case of ion exchange, fresh regenerative solutions may be required for subsequent use of the ion exchange resins, leading to significant running costs.
Removal of the nitrate ions using electrolysis is also known. For example EP-A-291,330 describes a process for treating ground water containing nitrates, which comprises contacting the water with a ion exchange resin and regenerating the resin with a regenerant, wherein the spent regenerant is subjected to electrolysis. The regenerant may, for example, comprise bicarbonate, chloride or sulphate ions. The electrolysis is carried out in an electrolytic cell containing an anode and a cathode. The material of each electrode is platinised titanium, nickel, stainless steel, copper or graphite. The nitrogen gas which in evolved can simply pass into the atmosphere.
U.S. Pat. No. 3,542,657 describes a method of converting an alkali metal nitrate to an alkali metal hydroxide by passing a solution of the nitrate through an electrolytic cell in which a direct current is imposed between the anode and cathodes in the cell, thereby producing oxygen gas at the anodes and alkali metal hydroxide at the cathodes. Nitrogen gas is also produced a the cathodes. A bipolar cell is preferably used in which the cathodes are copper, lead, tin, iron, silver, cadmium, platinum, cobalt, nickel and alloys thereof or coatings of these on the other metals.
The present invention seek to provide a further process for removing nitrate ions from an aqueous solution thereof using an electrochemical cell.
The present invention provides a process for removing nitrate ions from an aqueous solution thereof which comprises passing the solution through an electrochemical cell comprising at least one anode and at least one cathode and passing a current therebetween, wherein the cathode surface(s) comprise rhodium metal.
It has surprisingly been found that the electrical efficiency of the electrochemical cell wherein the cathode surface(s) comprise rhodium metal is surprisingly better than that of other cells containing cathode surfaces comprising, for example, platinum or nickel.
For example, it has been found that in a conventional bipolar electrical cell in which the anodes and cathodes are both made of titanium coated with a mixture of ruthenium dioxide and titanium dioxide, the electrical efficiency for destroying nitrate ions in a bicarbonate solution is about 40%, with about 12% of nitrate ions being reduced in a single pass through the cell. In the same cell fitted with anodes and cathodes made of nickel, the electrical efficiency is about 35%. However, if a cell comprising anodes made of titanium coated with a mixture of ruthenium dioxide and titanium dioxide and cathodes made of titanium electroplated with rhodium, the electrical efficiency is about 49% and about 24% of nitrate ions are reduced in a single pass through the cell. The exact values will, of course, depend on the cell dimensions and operating conditions.
The process of the present invention can be used to remove either partially or completely, nitrate ions from any aqueous solution thereof. However, it is preferred that the aqueous solution is one which has been obtained from the regeneration of an ion exchange column. The ion exchange column may, for example, have been used to purify water, especially ground water or surface water, which contains nitrate ions. The nitrate ions may be present in the solution treated by the ion exchange column in a concentration of, for example, from 15 to 1000 ppm, preferably 15 to 500 ppm. The ground water or surface water which may be treated can subsequently be used as drinking water. The maximum permitted nitrate level in drinking water is generally limited to 50 ppm (as nitrate) as a global standard.
The aqueous solution of nitrate ions treated in the electrochemical cell may also comprise further anions, for example, hydroxide ions, bicarbonate ions and chloride ions. It may also contain cations such as hydrogen, sodium or potassium. The electrochemical cell itself is well known and is described, for example, in U.S. Pat. No. 3,542,657. However, it is essential that the cathode surface(s) comprise rhodium retail.
The cathode may, for example, simply consist of rhodium metal, although this is expensive. Accordingly it is preferred to coat a cathode substance with rhodium metal, for example by electroplating. The thickness of the coating is desirably 0.1 xcexcm to 0.75 xcexcm, for example 0.5 xcexcm to 0.75 xcexcm.
The cathode substrate may, for example, comprise a metal such as titanium. It nay also comprise an intermediate coating layer under the rhodium metal coating, for example to facilitate the rhodium coating process and to reduce the amount of rhodium used in view of its expense. Thus, for example, the cathode substrate may comprise titanium or titanium coated with titanium dioxide, ruthenium dioxide, iridium. dioxide and/or gold. Many cathode substrates are commercially available.
The anode may be any appropriate anode. Suitable anodes are known to those skilled in the art. The anode surface may be coated with metals or metal oxides which promote the generation of chlorine over oxygen evolution. Thus, for example, the anode may comprise a metal such as titanium, optionally coated with a metal or metal oxide. Examples of metals are platinum, ruthenium and iridium. Examples of metal oxides are titanium dioxide, ruthenium dioxide, oxides of platinum and iridium and mixed oxides of these metals. Advantageously, the anode surface does not comprise rhodium metal so as to avoid undesirable back-reactions.
In a bipolar cell configuration one side of an intermediate electrode functions as a cathode, whereas the other side functions as an anode. In this case the cathode side is coated with the rhodium metal.
Desirably all of the cathode surfaces in the electrochemical cell comprise rhodium metal. However, this is not an essential feature and only some of the surfaces need comprise rhodium metal. Desirably at least 75% and preferably 100% of the cathodes comprise rhodium metal on their surfaces. Desirably the entire surface of each cathode comprises rhodium metal.
The electrochemical cell is suitably operated at elevated temperature, i. e. at a temperature above room temperature (20xc2x0 C.). For instance it may be operated at a temperature of at least 60xc2x0 C., Preferably 60xc2x0 C. to 70xc2x0 C., more preferably at a temperature of about 65xc2x0 C. We have found that the efficiency of the reduction of nitrate ions to nitrogen gas increases with increasing temperature. Suitable heat exchange means may be provided to heat the aqueous solution entering the electrochemical cell using the heat of the aqueous solution exiting the electrochemical cell.
The decomposition of the nitrate ions in the electrochemical cell follows the formulae given in U.S. Pat. No. 3,542,657. Thus the decomposition of nitrate ions in the electrochemical cell is balanced by the formation of hydroxide ions. If the aqueous solution exiting the electrochemical cell is used for any further purpose, the solution may be further treated to remove the hydroxide ions if this in appropriate, for example by the addition of an acid such as hydrochloric acid to neutralise the hydroxide ions.
Desirably the process for removing nitrate ions according to the present invention is used for the removal and destruction of nitrate ions from a solution obtained from the regeneration of an ion exchange column. The ion exchange column can itself have been used to remove nitrate ions from water such as ground water or surface water.
Thus the present invention can also provide a process for the removal and destruction of nitrate ions from water which comprises:
i) passing the water through an ion exchange column (a) containing nitrate selective anion exchange resin to exchange the nitrate ions with bicarbonate and/or chloride ions; and
ii) destroying the nitrate ions and regenerating the ion exchange column (a) by:
a) reliving from the ion exchange column (a) any cations which form insoluble hydroxides or carbonates;
b) passing an aqueous solution comprising bicarbonate and/or chloride ions through the ion exchange column (a) to exchange the nitrate ions with bicarbonate and/or chloride ions;
c) passing the solution from step (b) through an electrochemical cell to convert the nitrate ions to nitrogen gas by a method as defined above;
d) replenishing the solution from step (c) by adding bicarbonate and/or chloride ions thereto; and
e) recycling the solution from step (d) to step (b).
The water is passed through the ion exchange column (a) to exchange nitrate ions with bicarbonate and/or chloride ions. The water may, of course, undergo pre- or post-processing if other impurities, such as organic materials, are present.
When the water is passed through the ion exchange column (a) the nitrate ions in the water are replaced with bicarbonate and/or chloride ions and the anion exchange resin is loaded with nitrate ions. The anion exchange resin is a nitrate selective resin which exchange nitrate ions with bicarbonate or chloride ions. Examples of suitable resins are Purolite A520E supplied by Purolite International Limited and IMAC HP555, supplied by Rohm and Haas Limited.
Eventually the ion exchange resin will become fully loaded with nitrate ions. At this time the nitrate ions must be removed from the anion exchange column (a) and destroyed and the ion exchange resin regenerated so that the anion exchange column (a) can be used again in the process.
As an initial step any cations which form insoluble hydroxides or carbonates must be removed from the ion exchange column (a). These are mostly Mg++ and Ca++. The cations may be removed by any appropriate method. Preferably, however, they are simply displaced by passing a volume of softened water through the ion exchange column (a).
An aqueous solution comprising bicarbonate and/or chloride ions is then passed through the ion exchange column (a) to exchange the nitrate ions with bicarbonate and/or chloride ions. A suitable solution comprising bicarbonate ions is a solution comprising sodium or potassium bicarbonate. A suitable solution comprising chloride ions is a solution comprising sodium or potassium chloride or hydrochloric acid. Desirably the solution comprises either chloride or both bicarbonate and chloride ions.
The solution comprising bicarbonate ions generally comprises up to 1M bicarbonate ions, preferably from 0.75 to 0.9M bicarbonate ions. The solution comprising chloride ions generally comprises up 2M chloride ions, preferably from 1 to 2M chloride ions. When both bicarbonate and chloride ions are used, the solution generally comprises up to 1M, preferably 0.75 to 0.9M, bicarbonate ions and up to 2M, preferably 0.3M to 2M, chloride ions.
After the aqueous solution has passed through the ion exchange column (a) it comprises nitrate ions and either or both of bicarbonate and chloride ions. The solution is then passed through an electrochemical cell to convert the nitrogen ions to nitrogen gas in accordance with the method of the present invention defined above.
The solution exiting the electrochemical cell is recycled bask to the ion exchange column (a). However, it is necessary to replenish the solution by adding bicarbonate and/or chloride ions thereto. In order to add bicarbonate ions further sodium bicarbonate or potassium bicarbonate may, for example, be added. However, this is not preferred since it allows a build-up of hydroxide ions in the regenerant which leads to a rise in the level of hydroxides, such as calcium hydroxide and magnesium hydroxide, which precipitate out in the column during normal processing of water. Therefore it is more desirable to bubble carbon dioxide gas through the solution to convert the hydroxide ions, produced as a by-product from the reduction of nitrate ions, to bicarbonate ions. To replenish the chloride ions it is generally appropriate simply to add sodium or potassium chloride or hydrochloric acid.
Once the anion exchange resin has been regenerated the ion exchange column (a) may be used again for the removal of nitrate ions from water containing nitrate ions. Thus steps (i) and (ii) may be, if desired be repeated at least once. Of course, in practice, the process may be repeated again and again many times.
The water being treated by the ion exchange column may contain impurities apart from nitrate ions. In order to ensure that these anions do not adversely affect with the process of the present invention, the anion exchange resin is a nitrate selective anion exchange resin so that it exchanges the nitrate ions in water preferentially over other anions such as sulphate and phosphate.
The process of the present invention can, therefore, be carried out on water which also comprises other anions. Thus, for instance, especially when the water also comprises, for example, sulphate, chloride or phosphate ions, the process may additionally comprise i) passing the outflow from the ion exchange column (a) through an ion exchange column (a) containing an ion exchange resin to exchange any nitrate ions in the outflow with bicarbonate and/or chloride ions until substantially the nitrate concentration in the outflow from ion exchange column (a) is equal to the nitrate concentration in the inflow to ion exchange column (a); and ii) removing the ion exchange column (a) from the flow of water by passing the water directly into the ion exchange column (b).
The nitrate concentration can be measured continuously or non-continuously by any one of the methods known in the art.
The above embodiment ensures that the water may be continuously treated. Thus the removal of nitrate ions, and hence the treatment of the water flow, does not have to be stopped while a single ion exchange column is regenerated. Desirably the ion exchange column (a) is regenerated and the outflow from the ion exchange column (b) is then optionally passed through another ion exchange column. This may be the regenerated ion exchange column (a).
Another possibility is to have more ion exchange columns. If the total number of columns is n, the water is generally only passed through nxe2x88x921 columns at a time. Thus, for example, 3 or 4 ion exchange columns may be used, but with the water only passing through 2 or 3 of them at a time. The remaining column will have been taken out of the water processing circuit and will be in the process or being regenerated. Thus, if 3 columns are used, a snap shot of the operation could show 2 columns processing water separately in parallel and the third column being regenerated, or in stand-by (i.e. ready to be used in series after one of the other columns).
Thus, as one embodiment of the invention, initially the water passes through ion exchange column (a) until nitrate levels in the effluent are seen to increase, indicating breakthrough. Ion exchange column (b) is then added in series to ion exchange column (a) (or it may previously have been attached in series) and water passed through both columns until the nitrate level in the effluent from column (a) is seen to be substantially identical to the level in the water fed into ion exchange column (a), indicating maximum nitrate absorption on column (a). Water is then passed only through column (b) and column (a) is regenerated. After ion exchange column (a) has been regenerated, the water continues to be passed through ion exchange column (b) until nitrate levels in the effluent from this column are seen to increase, indicating breakthrough. Ion exchange column (a) (or another ion exchange ion column (c) if ion exchange column (a) is still being regenerated, for example) is then added in series to ion exchange column (b) and water passed through both columns until the nitrate levels in the effluent from ion exchange column (b) is seen to be substantially identical to the level in the water fed into ion exchange column (b), indicating maximum nitrate absorption in ion exchange column (b). Water is then passed only through ion exchange column (a) (or ion exchange column (c)) and ion exchange column (b) is regenerated.
Of course, this embodiment may be modified so that ion exchange columns are taken out of the water flow before they have maximum nitrate absorption if desired. This arrangement may be modified by including more ion exchange columns in the series. In general three or four ion exchange columns are used in a xe2x80x9cmerry-go-roundxe2x80x9d arrangement.